Aluminum base alloy casting



United States Patent ALUMINUM BASE ALLOY CASTING Herbert C. Rutemiller, Cleveland, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. ApplicationMay 27, 1958 Serial No. 738,026

7 Claims. (Cl. 148-2191) This invention relates to an aluminum-magnesium alloy type of casting and it is more particularly concerned with an improvement in the alloy which increases the resistance of the casting to stress corrosion cracking.

Binary aluminum-magnesium alloys have been used for many years in both cast and wrought form because of their strength and resistance to corrosion. The commercial alloy composed of aluminum and magnesium has been widely employed in making high strength castings. To attain the highest strength the casting must receive a solution heat treatment and then be quenched from the heat treating temperature. It has been found that unless the casting is quenched under controlled conditions it becomes susceptible to stress corrosion cracking after a period of natural aging. Stress-corrosion cracking is a form of corrosion which is stimulated if the article is under either internal or external tensile stress while exposed to a corrosive environment. Quench: ing practices designed to eliminate or substantially reduce any such corrosion are disclosed in V United States Patents 2,157,150 and 2,390,238. The first patent describes the use of hot oil in place of water as a quenching medium and the second patent discloses immersing the heat treated castings in boiling water for a short time and then removing the castings from the quenching medium before their temperature reaches 400 F. While these practices are eifective if properly employed they do involve very close control and this may be difiicult under some commercial operating conditions.

Another characteristic of the solution heat treated and quenched castings of the aluminum-10% magnesium alloy is the tendency to lose some ductility upon prolonged -standing at room temperature as manifested by a reduction in elongation values. Such a changemay adversely affect the service life of the castings under some conditions.

The problem of making aluminum-magnesium alloy castings which combine a high strength with substantial freedom from stress corrosion cracking has been the subject of much study and investigation. It is an object of this invention to provide a casting of an aluminummagnesium type of alloy which when solution heat treated does not require special quenching practices. Another object is to provide a casting of an aluminum-magnesium type of alloy that is substantially free from susceptibility to stress corrosion cracking in the solution heat treated and quenched condition. A further object is to provide an aluminum-magnesium alloy casting that undergoes little if any change in ductility after solution heat treatment and quenching. A particular object is to provide a sand casting of this type of alloy.

These and other objects and advantages are realized in a casting composed of an alloy consisting essentially of aluminum, 7 to 9% magnesium, 1.0 to 2.25% zinc and 0.1 to 0.2% copper. If desired, from 0.1 to 0.3% manganese may be added to improve the strength of the casting. treated in the normal manner and then quenched in r 7.4 and 8.4%.

2,922,731- Patented Jan. 26, 9

water at a temperature between 180 and 212 F., the castings remaining in the water until they have attained substantially the temperature of the quenching medium. There is no need for interrupting the quench by removing the castings from the boiling water before reaching the temperature of the water in order to minimize susceptibility to stress corrosion cracking. In the solution heat treated and quenched condition the castings have a minimum tensile strength of 42,000 p.s.i. and a minimum yield strength of 25,000 p.s.i. Furthermore, the castings are substantially free from susceptibility to stress corrosion cracking. Also, upon standing at room tem perature for long periods of time there is substantially no change in the elongation 'values of the castings.

To attain the properties referred to above it is necessary that the proportions of the individual elements be strictly observed. For example the desired tensile strength is not obtained if less than 7% magnesium is present While on the other hand susceptibility to stress corrosion cracking is encountered if the magnesium exceeds 9%; Preferably the magnesium content should be between At least 1% zinc is essential to obtain freedom from stress corrosion cracking, but on the other hand more than 2.25% is not beneficial in maintaining a substantially constant elongation value over a long period of time. In preferred practice the zinc is employed in amounts between 1.2 and 1.7%. At least 0.1% copper is necessary to achieve the properties of the alloy, the desired results not being obtained if less than this minimum amount is present, whereas more than the maximum of 0.2% has an adverse elfect upon the performance of the castings.

Manganese is a beneficial element which can be added to improve the strength of the castings but it should not be employed in amounts exceeding 0.3% and at least 0.1% is required if any benefit is to be realized.

In addition to the elements mentioned above it may be desirable to add one or more of the elements boron, titanium, and beryllium in very small amounts. Boron and titanium are recognized as grain refining elements in the aluminum base alloy art. In the present instance boron Castings of this alloy can be solution heat may be employed in amounts of from 0.001 to 0.02% and titanium in amounts of 0.03 to 0.20%. To minimize oxidation of the molten metal "beryllium may be employed in amounts of 0.001 to 0.01%.

To achieve the results described above it is also necessary to restrict the iron and silicon impurities, the maximum iron permitted being 0.30% and the maximum silicon 0.20%. It is to be understood that traces of other elements may be present as impurities such as those associated with primary aluminum. The melting and casting practices employed should follow those used heretofore in handling the aluminum-magnesium commercial compositions.

Although the improved alloy is particularly adapted to the production of sand castings, however, it can also be used in making castings in permanent molds whether cast under gravity or pressure conditions.

To develop the desired strength and resistance to stress corrosion cracking the castings must be subject to a solution heat treatment within the temperature range of 825 to 940 F. To achieve the best results, however, it is desirable to perform this operation in two steps, the first consisting of heating the casting for at least 6 hours at 825 to 840 F. and thereafter, without intermediate cooling, raising the temperature to 875 to 940 F. and hold the castings within the range for a period of at least 4 hours. Generally, the soaking period within the prescribed temperature ranges need not extend beyond 12 hours. tion of all soluble constituents. Following the solution heat treatment the castings should be quenched in water The two step treatment insures maximum soluat a temperature between 180 and 212 F. and held therein until they have reached the temperature of the water. It is preferred, however, that the water he at approximately its boiling point. Since the foregoing temperature range includes the boiling point of water, the quenching medium may be conveniently referred to as being boiling water. It is also to be understood that the water may contain the usual impurities and any substances which accumulate from continued use of a given quench bath. Sand castings treated in this manner have a typical tensile strength of 48,000 p.s.i., a yield strength of 26,000 p.s.i. and an elongation of 116%.

The solution heat treated and quenched castings should not be subjected to any subsequent artificial aging or percipitation hardening treatment, since such treatment has an adverse effect upon the strength. Some natural aging may occur over a long period of time but the effect upon the ductility is insignificant.

The improved resistance to stress corrosion cracking obtained with the new composition is illustrated in the following examples. Three alloys were used in making sand cast tensile test bar specimens, (A) commercial aluminum-% magnesium alloy, (B) aluminum, 8% magnesium, 1.5% zinc and 0.15% copper and (C) aluminum, 8% magnesium, 0.15% copper, 0.25% manganese and 1.5 zinc. The test bars of the aluminummagnesium alloy were heated for 16 hours at 825 F. and quenched in water at its boiling point, the bars not being removed therefrom until they had attained the tem perature of the water. The test bar specimens of the alloys (B) and (C) were heated for 8 hours at 825 F. and immediately thereafter heated for 8 hours at 925 F. following which they were quenched in water at its boiling point in the same manner as the specimens of the first alloy. Groups of specimens of the three alloys were subjected to one year of natural aging and then exposed to an accelerated stress corrosion test lasting for a maximum of 14 days which consisted of placing them under a stress approximately equal to 75% of their yield strength and immersing them in a standard 5.3% sodium chloride and 0.3% hydrogen peroxide aqueous corroding solution. The bars of alloy (A) broke in one day while bars of alloys (B) and (C) remained intact over the entire test period of 14 days.

Other specimens of alloys (A) and (C) were tested shortly after quenching and after standing for 1 year to determine any changein elongation values. It was found that the average elongation of the alloy (A) test bars had dropped from 21 to 13% while that of the. alloy (B) specimens changed only 2% in the same period of time, the elongation decreasing from 20 to 18%.

Having thus described my invention and certain embodiments thereof, I claim:

1. A solution heat treated casting of an aluminum base alloy consisting essentially of aluminum, 7 to 9% magnesium, 1 to 2.25% zinc, and 0.1 to 0.2% copper, said alloy containing a maximum of 0.30% iron and 0.20% silicon as impurities, said casting having a structure produced by solution heat treating at 825 to 940 F. and quenching in water at 180 to 212 F. to the temperature of the water, said treated casting being characterized by having a minimum tensile strength of 42,000 p.s.i., a yield strength of 25,000 p.s.i., said casting also beingsubstantially freefrom susceptibility to stress corrosion cracking.

2. Acasting of an alloy according to claim 1 containing from 7.4 to 8.4% magnesium, 1.2 to 1.7% zinc and 0.1 to 0.2% copper.

3. A casting of an alloy according to claim 1 which also contains 0.1 to 0.3% manganese.

4. A casting of an alloy according to claim 1 which also includes at least one grain refining element of the group consisting of boron and titanium in amounts of 0.001 to 0.02% boron and from 0.03 to 0.20% titanium.

5. A casting of an alloy according to claim 1 which also contains from 0.001 to 0.01% beryllium.

6. A method of solution heat treating and quenching a casting of an aluminum base alloy consisting essentially of aluminum, 7 to 9% magnesium, 1 to 2.25% zinc and 0.1 to 0.2% copper comprising heating said casting to a temperature between 825 and 840 F. and soaking within said temperature range for a period of 6 to 12 hours, thereafter heating said casting, without intermediate cooling to room temperature, to a temperature between 875 and 940 F. and soaking within said temperature range for 4 to 12 hours, and finally quenching said casting in water at a temperature of 180 to 212 F. and holding it therein until it reaches the temperature of the water.

7. The method of claim 6 wherein the quenching medium consists of water at its boiling point.

References Cited in the file of this patent UNITED STATES PATENTS 2,245,167 Stroup June 10, 1941 2,336,512 Stroup Dec. 14, 1943 FOREIGN PATENTS 7 1,004,523 France Mar. 31, 1952 

1. A SOLUTION HEAT TREATED CASTING OF AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM, 7 TO 9% MAGNESIUM, 1 TO 2.25% ZINC, AND 0.1 TO 0.2% COPPER, SAID ALLOY CONTAINING A MAXIMUM OF 0.30% IRON AND 0.20% SILICON AS IMPURITIES, SAID CASTING HAVING A STRUCTURE PRODUCED BY SOLUTION HEAT TREATING AT 825* TO 940* F. AND QUENCHING IN WATER AT 180 TO 212*F. TO THE TEMPERATURE OF THE WATER, SAID TREATED CASTING BEING CHARACTERIZED BY HAVING A MINIMUM TENSILE STRENGTH OF 42,000 P.S.I., A YIELD STRENGTH OF 25,000 P.S.I., SAID CASTING ALSO BEING SUBSTANTIALLY FREE FROM SUSCEPTIBILITY TO STRESS CORROSION CRACKING. 